Step 1: Preheat your oven to 350°F (175°C). Grease and flour an 8x8 or 9x9 inch square baking pan, or line with parchment paper.
Step 2: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, whisk together the melted butter (or oil) and granulated sugar until well combined.
Step 4: Add the egg and vanilla extract to the wet ingredients, whisking until smooth and creamy.
Step 5: Stir in the yogurt (or sour cream) until fully incorporated.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or spoon until just combined. Be careful not to overmix; a few small lumps are fine.
Step 7: Gently fold in the diced fresh strawberries. Ensure they are evenly distributed.
Step 8: Pour the batter into your prepared baking pan and spread it evenly. If desired, scatter a few extra strawberry slices on top for garnish.
Step 9: Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The top should be golden brown.
Step 10: Let the cake cool in the pan on a wire rack for at least 15-20 minutes before slicing and serving. It can be enjoyed warm or at room temperature.